commit-graph: stop using `the_hash_algo`

commit-graph: stop using `the_hash_algo`

Stop using `the_hash_algo` as it implicitly relies on `the_repository`.
Instead, we either use the hash algo provided via the context or, if
there is no such hash algo, we use `the_repository` explicitly. Such
uses will be removed in subsequent commits.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>